Marti and Venema [1]} introduce Moss liftings,
predicate liftings that are constructed from functor presentations
with the help of a generalized form of the nabla operator, extending
an earlier construction for weak-pullback preserving functors by Kurz
and Leal [2]}. Recall that for a
weak-pullback-preserving functor \(T\) , Moss' [3]} classical
nabla operator \(\nabla : T\mathcal {Q} \Rightarrow \mathcal {Q} T^\mathsf {op}\)  is the
natural transformation defined by
\(\nabla (\Phi ) = \lbrace  t\in TX \mid ( t, \Phi ) \in \overline{T}(\in _X)\rbrace .\) 
